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Number on your eedback card

THE 2 MHz ALL MOD

Designed for the dedicated home brewer.

by Robert E. loom W6YUY

B cause of the increased interest andactivity in 220 MHz single sideband

and the positive response to my Two-MeterTransverter project in July 198773 (pps 32-43), I have now designed an improved, 220M H z v e r s i o n o f t h e t r a n s v e r t e r. T h i stransverter is capable ofFM repeater opera-

tion as well as CW and SSB, and it s muchmore soph i s t i ca t ed than the usua l FMtransceivers and hand-helds on the market.All frequencies on this unit can be set withdirect read-out to the nearest 10 Hz. Ofcourse, DX simplex operation on both FMand SS B is one of its m ost exciting features.

Like the earlier unit, this is an all-modetransverter specifically designed to interfacewith the Kenwood TS -940sHF transceiver.It will also work with other full-frequencycov erag e t ransceivers equipped wi th atransverter access plug. T he transverter has aCW output of 3-% watts. I will cover in a

later article a 220 MH z DM OS linear amplifi-er with a pow er output of 60 watts.

Why No t PC Cards?The construction of this unit uses point-to-

point wiring in a soldered PC-card structurethat provides several benefits.

1. A printed circuit board would require foilcircuits on both sides, making double-deckconstruction impossible.

2. Compartmentalizing would require a sepa-rate P board fo r each stage or section.

3 . A PC card would require more space be-cause of restrictions on parts placement.

With this type of construction you can usethe walls of the compartment as well as thefloor for mounting parts.

4. Exact duplication of components is notrequired.

5. Excellent interstage isolation.6. More freedom and ease of modification

than normal printed circuit board tech-niques.Your TS-940 transceiver must be modi-

fied, of course, to work with the 220 MHztransverter, because 222 to 225 M Hz will bemapped into 22 to 25 MHz and the unmodi-fied transceiver will not transmit outside the

ham bands. (Obviously, it is not legal totransmit with the modified Kenwood outsidethe amate ur bands authorized by your class oflicense.)

Tak e I t S tep-by-s tepRealizing the apprehension you may feel

when looking at a complex project,I provid-ed , in Figures 5 and 6, drawings of the layoutof the major parts. By showing the approxi-mate placement of coils, tuning capacitors,and transistors, I hope to simplify the place-ment of the rest of the components for you.Since you are building this unit step-by -step,

you won t have to place the shields betweenstages in exactly the locations shown in thedrawings. Also, your components may be ofdifferent sizes, but the compartm ents shouldhave enough sp ace in most cases.

THE HOUSING

Case and Module Fabr ica t ionThe 220 MHz transvelter is a one-piece

module with a housing made up of double-sided PC board m aterial. T he top of the chas-sis contains the receiver and oscillator chain;the bottom has the transmitter and control

circuitry.The outside dimensions of the transverterassembly are 8-5 s x 4- 5 4x 2- 7 . The heightof the partitions is 1/16 less than the depth ofthe compa rtment, so the top and bottom cov-ers fit flush with the side panels. Similarly,the front and back panels are% wider than

Figure 1. Schematic ofthe receiver portion o the 222 225 MHz transverter.

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T O R E L AY R Y 3V O LT A G E C O N T A C T S

Figure 2. Schematic of crystal oscillator circuit 100 MHz) and doubler.

the main circuit board chassis to allow for the1 / 1 6 side panel thickness.

Dra w a pencil line down the m iddle of each

side piece to help in locating the main centerboard that separa tes the top from th e bottom.Start by tack-soldering just inside each e nd,making su re that the sides are at exact rightangles with the center board. Then tack theend pieces into position. When the box issqua re, solder the corners and then seam-sol-der the sides from the inside.

Figures and 6 are parts layout drawingsshowing the individual compartment sizes,

/16 ' ' and do not allow for the /16 thick-ness of the separators. These dimensions areflexible and can be used as an indication ofhow much room you need and how much

space you have left as you go along .The com partment separators are also made

up of double-sided PC card stock and con-struction is serial: you complete one stageand install a separator before going on to the

next stage. This way, you won't be caughteither short or long on space. Dril l al lfeedthrough holes in the co mpartm ent sepa-rators before soldering the separators in-drilling afterward s s difficult

Single-sided PC stock is used for the termi-nal mounting pads to which compon ent leadsare soldered . If all the PC material you haveis double-sided, you can remove the foil fromone side by holding the material in a benchvise, heating one side gently with a propanetorch, and pulling off the foil with pliers.

Make up a supply of single-sided strips-l i , 5 /32 , and ide-and snip off pieces

as needed with diagonal cutters. Usinga sin-gle drop of Eastman 91 cement (Crazy

Glue), position these pads w here needed withlong-nose pliers. Four seconds late r, you cansolder to the pad. Should you decide that the

pad is not in exactly the right position, youcan remove it by prying with a pocket knife orsmall sharp screwdriver. Do not re-use a pad,as it will not hold securely the second time.

The unit will operate well without the cov-ers in place. Ho wever, when the top cov er isremoved exposing the receiver's pre-selectorstages, the unit will pick up strong signalseven if it isn't connected to an antenna.Rather than secure the covers permanently,you can hold them in place with transpare nttape for easy access to the circuitry.

Panel Hardware

You can drill the holes in the front and backof the case either before or after the box is

JUNCTIONOLTAGE POINTSITH OTHERND

CONNECT TO CONTlCTS

Figure 3 220 MHz transmitter low-level amplifier section.

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L-4L-5L-6L-7L-8L-9L-10L-1 1L-12FB Ferrite BeadsQ - 1 4 - 3Q-4Q - 5 4 - 7

22 MHzReceiverMajor omponents Parts List and oil Data

(Small resistors, capacitors, and inductors not listed)1-7-pF quality miniature piston capacitors2-12 pF Johnson air variable,PC board ceramic3-35 pF ceramic variablesFour turns #16 or 18 tinned wire1/4 inside dia., apped at 1-l/4 turns19 turns #20 enameled wire on T-50-12Amidon core, or 21 turns #24enameled wire on T-37-12core4 turns #16 tinned wire, wound on #3drill (0.212 ), ll four turns spaced 1-1/2wire diameterstapped at h and 2-l/2 turnstapped at 2 turnstapbed at 12 and 3 turnstapped at 2-1/2 turnstapped at 3 turnsno tapstapped at 3 turns22 turns #28 enamel wire on T-25-6Amidon powdered-iron oroid coreSame as L 1 1 but 19 turns101 size of 43 ,64, or 75 material; typical Amidon designation, FB-43-1012N5485 or 2N5486 (or equivalent) JFETs40673 or 40673A dual gate MOSFET2N918 transistor2N51092N5485 or 2N5486 JFETs

~ o t b :ll resistors used in the project areY4 watt 1100/0 unless stated otherwise.

Table 1

assembled. The front panel has two BNC The oscillator coil L-13 in Figure 2 is tunedbulkhead connectors on the left edge. One is by a network of fixed and variable capacitors.fo r received sign al in and the o ther is fo r The variable capacitor is tuned for resonance,transmitted sign al ou t, to interface with the while the parallel and series divider providespower amplifier. An LE D shows when power the required drive level to the frequency dou-is applied. The three-pole, double-throw bler. (With the proper selection of capacitors,miniature switch and eight-pin male mikebulkhead connector complete the panel. Theeight-pin connector couples to the TS-940interfacing cable. The back panel has afour-section miniature barrier strip for sourcepower t o the transverter.

IR UIT SE TIONS

Receiver onverterThe receiver portion (shown in Figure 1)

has three JFET pre-amplifier stages. Dualgate MOSFETs would provide twice thegain, but much less stability and more noise.The bandwidth of these stages is set by posi-tioning the taps on the R F coils (see the CoilTapping sidebar). A tap placed closest to thetop or high impedance point of the coil pro-

.

vihes heavy loading, driving theQ of thestage down for a broader bandwidth. Thelower the tap p oint, the higher the circuitQ,and the more selectivity and gain per stage.

Because the transverter is not retuned d ur-ing operation, the bandwidth or bandpass dis-cussed here is the total bandwidth over whichthe transverter will work. The actual opera-tional selectivity is provided by the TS-940being used as a tunable IF.

Each of these stages provides about 12 dBof gain, with 15 dB the maximum. Aband-pass and imag e frequency rejection filter pre-cedes the preamplifier.

rystal OscillatorThe oscillator is a variation of the Butler

design-one of my favorit es. The oscillator

we can retain the correct parallel [product-over-the-sum] capacity, and we can select acorrect drive level for the frequency dou bler.In addition, we reduce loading on the oscilla-tor output circuit.)

The selection of coil tap on L-13 providesthe oscillator feedback voltage required forstable oscillation. T he 100 MH z crystal oscil-lator feeds the doubler, which retains about66 of the oscillator's energy. Using a 100 -MHz crystal and a doubler, the 200 MHz ismuch cleaner than it would be if you used atripler. A simple doubler circuit provides therequired 200-MHz output frequency, whilethe amplifiers with double-tuned bandpasscircuits clean up the frequency from the out-put of the doubler stage.

To further clean up the 200 MHz signalbefore it enters the receiver mixer, we add athird amplifier and bandpass filter stage. Afourth, and similar amplifier stage providesthe heavier signal required fo r the transmittermixer. The double-tuned bandpass filters inthese amplifier stages clean up whatevergarb age is generated in the early stages of theoscillator chain.

Transmitter MixerMu ch research wen t into the determination

of transmitter mixer circuitry. Both activeand passive, double- and single-balancedtypes were studied. I selected the single-bal-anced, active JFET circuit after comparingmajor characteristics such as dy namic range,suppression of intermodulation products, andcross-modulation effects. FETs were select-ed over bipolar transistors for their in herenttransfer characteristics approaching a squ arelaw response, thus providing a reduction ofthird overtone products. H armonic distortionand cross-modulation effects are third-orderdependent, and are greatly reduced when us-ing FET s in a balanced mixer.

For RF amplification, use the followingguide to set up the DV-1205s V-MOSFETstage 4-14 in the main transverter unit (seeFigure 3 . The level of drain current andultimate power output of the stage is a func-tion of gate voltage controlled by the 1Okpotentiometer. Four volts gives a drain cur-rent of 200 d ix volts yields 40 0d ndseven volts sets up 600m.4. If you don't getan increase in power output with increaseddrain current, you don't have enough drivepower and it isn't economical to increase thedrain current further. Since this project hasthree MOSFET linear amplifier stages, setthe output levels for only what you require.On the other hand , if the drive is much greaterthan you need, don't worry about blowing theFETs, since you cannot hurt them byover-driving.

ontrol ircuit FunctionRefer to Figure 4 and Table4. Plugging the

DIN plug into the TS-940 mechanicallyswitches the TS-940's input circuitry todesensitize it to H F signals. At the sam e time,

'oTgOT LCHED

TO P IN 4 ON 8-PIN CONN.

I N P U T13 TO 15 VOLTS

+ @ - @ -

SWITCHED13 TO 15 VOLTS

IXUIT ONLY1-

LOCATE JONESSTRIP ON REAROPPOSITETRANSMITTERSECTION

n a O PIN 5 ON 8-PIN CONN.

TO PIN 8 ON 8-PIN CONN.

I3 TO 15 VOLTS TO R ECEIVERCONVERTERR - F CIRCUITi ~Z ;ON THISONNECTIONSIDE

I N4 0 0 4 1 ~ 4 0 0 4 TH NUMBEROF PARALLELEDPINS DEPENDS ONCONTACT CURRENT CARRYING CAPACITY

NOTETO PIN.

THE S I TRIPLE POLE DOUBLECONN i .

THROW SWITCH IS LOCATED ONON TRANSMIT

THE PANEL ALONGSIDE THE 8-PINM A L E M I K E -T Y P E J A C K

, I N ' TO R E C EIV E W NP U T

-TO POWER AMPL INPUT

I 5VOLT LINE hi l J i N T E N N AWER LlNEPiR- N TRANSMIT

circiit is particuiarly suited for overtone I I

crystals, and, with light loading, is very stable. Figure 4 Control circuit wiring diagram

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With SW -1 ON , the first set of switch con-cts applies 13 to 15 volts from the inp utrminals of the barrier strip to the RY-2

rmature contact, providing voltage to all ofhe receiver circuitry and the oscillator chainexcept for the Q-9 stage). T he second set ofwitch contacts grounds the coil of RY-3 inhe TS-940S, closing the relay, disconnectinghe VFO, and disabling other LF circuits inhe TS-940.

When you press the microphone push-to-lk (PTT ) switch, the Kenwood puts 12 volts 50 mA on pin 2 of the 940's DIN plug,

ctivating the transverter RY-2, which in turnmoves the supply voltage from the receiver

nd activates transverter RY-3. RY-3 thenpplies supply voltage to the Q-9 output stageong with all the source voltages to the trans-itter section. In addition, plus voltage is

FULL OUTSIDE DIMENSIONS OF CASE 8 5/ 8 x 4 518 x 2 318 in.

1 /2 1 3 / 8 7 / 8 1 3 / 8 1 11116'1 13 /8

1 3 / 4

3/16

3 1 8 ' '

I I / 2 1 3 1 8 1 118 112 3/4 71.8* ( I 1 12 l 1 3 / 8 1

the barrier for the Figure 5 Majorpartspositioning approximatedimensions) or the receiver and oscillator, top section.near amplifier and antenna relay # I. Releas-

ng the PTT switches the transmitter and itsssociated circuitry off, and returns the unit cable with a Teflon dielectric. D on't use TS-940 receive.Toggling SW-1 to OFF, the arm of the

ird pole connects pin 5 with pin 8 on the40's transverter plug. This allows the TS-40 to operate in the normal manner on H F,ven though the transve rter is still connected .When purchasing the RY-2 control relay,

eep in mind that its 12-volt actuating voltageomes from the TS-940, and the maximumurrent available is 50 mA. Many 12-voltlays require more than twice this level. Th e

nit you select should have a field coil resis-nce of no less than 250 ohms.

S-940 Interface Cable30 cable connects the transverter to theVTR plug on the back of the TS-940 (seeable 4). Get the eight-pin DIN plug fromenwood. Don't try to buy this plug else-here, as the plug you get probably won'tt the Kenwood's jack. I purchased threeom other sources; no two were the sam e andone of them fit the TS-940. Just connect likeumbers on the DIN plug and the eight-pini k e j a c k . Yo u ' l l n e e d t h r e e r u n s o f

mall-diameter coax for pins5 , 7 , and 8. Use

RG-174-the dielectric can melt duringsoldering, causing a short. Pins 1 and3 are

s h i e l d g r o u n d . U s e a g o o d g r a d e o f#20-stranded between the two num ber 2 pinsand the two number 4 pins. Pin6 ALC on thetransceiver, is not used. Pin 6, by the way,is the center pin on the D IN plug , while pin8 is the center pin on the eight-pin mikeconnector.

Recall that the very act of inserting the DINplug into the 940 transverter jack disconnectsthe HF front end in the 940 to avoid leak-through of H F signals at what is now the firstintermediate frequency. In order to use theKenwood on HF when the transverter isplugged in, a switch in the transverter con-

nects pins 5 and 8 when the transverter isturned off.

Pin 2 provides 12 volts (at a maximum of50 mA during transmit for use as PT T in thet ransver ter. A ground on pin 4 by thetransverter disables the TS-940's pow er am-plifier. RFIIF input signals between 22 and25 MHz come from the transverter to theKenwood o n pin 5 w ith the outgoing (trans-mitted) signal at a level of about 10 0 mW onpin 7. Pin 8 is the TS-940's H F input, whichmust be connected to restore H F operation.

DIMEN SIONS SHOWN ARE APPROXIMATE FOR INSIDE OF THE PARTICULAR COMP ARTMENT

4 3/4 2 11/16

4 518 2 1 /4 1 7/16

Figure 6 Major parts positioning for the transmitter and con trol, bottom side.

To enable the TS-940 to transmit outsidethe regular ham bands, cut diode D-130 from

the circuit. The transceiver will then transmitanywhere between 1500 kHz and 3 0 MHz.D-130 is located on the unit B PC boardjust behind the LCD display on the panel.(The diode is identified, as a re other com po-nents, by screening on the PC board.) I re-moved the five screw s on the module contain-ing the board, but the board is still difficult toaccess and requires patience.A letter from aN e w Z e a l a n d r e a d e r o f m y p r e v i o u stransverter article suggested loosening thepanel to the point where it could be tiltedforward, exposing the diode, which is thencut free at one end using a diagonal cutter.

The V FO frequency for the tunable IF is 22 to25 M Hz. Thus, the TS-940 dial read-out for af requency o f 223 .259 .81 M Hz wi l l be23.259.81. The first digit 2 is understood,as all of the frequencies a re in the 200-MH zrange. F or example, 22 on the dial indicates222 MHz, plus all of the subsequent digits.Note that the frequency is read to the closest10 Hz, so be sure to zero-beat both the TS-940 crystal and the transverter oscillator tow w v .

Construction AidsFigure 4 shows the control circuit wiring

diagram. Schematics for the receiver andtransmitter sections, as well as separate ma-jor parts list for each section, are furnished.See Tables 1-3. In addition, see the photo-graphs of both top and bottom views of thetransverter.

If you follow the coil winding data, youshould not have a problem attaining reso-nance in any of the stages. Coil constructionhas been further simplified by using toroida lcores. These are readily available in smallquantities at under a dollar each fr omAmidonAssociates, Inc., 12033 Otsego St., North

HollywoodC

91607. Tell them you saw thecores mentioned in 73 Magazine.

Potpourri of Technical Construction NotesIn the bandpass filter in the 4-4 transistor

stages, L-10 is lightly coupled and at right7 Arnateur Radio January 989 2

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22 MHz Oscillator Chain and Transmitter MixerMajor Components Parts List

C-12 3-20 pF miniature air variable or ceramic disk'2-13, C-14 4-22 pF miniature air variable or ceram ic C-151-7 pF piston capacitorC-16, C-18 2-1 2 pF Johnson air variable used on PC b oardsC-17 2-10 pF John son similar to C-16RFC-1 4.7 pHRFC-2 3 turns on FB-64-5111 6-hole Amidon ferrite coreRFC-3 2.2 pHRFC-4-7 1.0p approx. 2 5 turns of # 28 enam eled wire on a %-watt resistor formL-13 7 turns #20 enam eled wire tappe d at 2 turns from cold end, wound on a

T-37-12 powdered-iron Amidon toroid c oreL-14 4tu rns # I8 bus wire 3/16 inside dia., 9/16 long, tapped at 3h turn at 3-3h

from cold endL-15 5 turn s #18 bus wire 3/16 ID x l/2 tapped at 1-l/z, 2% at 4 turnsL-16 6 turns #20 tapp ed at 3h at 5 turnsL-17 5 turns #18 bu s wire l/4 x 1/2 tapped at 1-1/2 turnsL-18 4 turns #I 8 bus wire lh x1/2 tapped at I-% turnsL-19 4 turns #16 bus wire wound on # 3 drill (0.2 12 ) apped at 1-1/2 turns from

cold end

Table 2 .

angles to the center of L-9. Do not place L-9and L-10 closer than % to each other. Thebandpass filters of both the Q-7 and 4 -8 cir-cuits also have their coils at right angles forminimum coupling. Here, the coupled coilsshould be placed at least I apart, so thatcoupling is primarily through the 5 pF and 1pF capacitors, respectively. Even at this dis-tance, there's some mutual coupling.

There are a number of places where RFchokes are called out. Most of these arewound using % - and %-watt resistors as thecoil forms. The following should assist in coilwinding.

A reactance vs. frequency, capacity, and

inductance chart or a cardboard reactanceslide rule are great tools. A grid dip meter isa n o t h e r. Yo u ' l l n e e d s e v e ra l 0 . 3 9 - p Hchokes. A 0.3 9-p H choke will resonate at 50MHz when shunted with a 27-pF capacitor.By using this point, you can locate othercapacitance and inductance values on thescales. You can m ake the 0.39 -pH unit with a%-watt resistor of 3000 ohms o r more. M akenotches at the ends of the resistor with ajeweler's file to prevent the wire from slip-ping. Close-wind 25 turns of #28-gage enam-eled wire on the resistor, and grid-dip with acapacitor to check the value.

You'll also need some 1.0-pH chokes.

22 MHz Unit RF ChainMajor Parts List for Transmitter

2-12 pF Johnso n air variable for PC boa rds5-25 pF Arco 400 series compression5-35 pF Arco 400 serie s compression4-40 pF Arco 400 serie s comp ression6-60 pF Arco 400 serie s comp ression10-80 pF Arco 400 serie s compression

2N51 09 transistor2N3866 transistor with top-hat h eat sinkMIA COM PHI RF power MOSFET DV-1 205s 5 watt5 turns #18 bus wire, wound on #3 drill, 9/16 long, tapped at72 and

from cold endSa me a s L-22, tapped at1/2 and I-% urns3 turns #18, 0.3 diameter spac ed 2 wire diameters7 turns #2 8 wound on a 6 2 or 68 ohm lh watt resistor5 turn s #18 bus wire on #3 drill, 911 6 long

turns #18 bu s wire on #3 drill, space d 3 wire diam eters L-28, L-303h of#16 tinned b us wire bent into a hairpin 113 diam eter

8 turns #22 Teflon cove red wire, close wound 1/4 diam eter'14 wide strip of sing le-sided PC board m aterial, epox ied as though it were

anoth er pad 0.66 long

As in L-29, but 1/2 longPart of Q-18 ,2N 386 6 bias circuit 570 ohm, m ade of a 1k ohm and 1.3k ohm%-watt resistors paralleled

The se should resonate at 16 MHz whenshunted with 100pF. Fill a %-watt resistorwith #31 wire. Wire size is important. Notethat 25 turns of #28 wire on a %-watt formalso makes the same 1-pH value. As an addi-tional aid, note that a value of 0.18 pH res-onates at 70 MHz when shunted with 27 pF .

Refer to the dimensions of stage compo-nents. Notice that on the receiver side, acompartment of 1% has only a few compo-nents. Th is compartment originally had a re-dundant stage. Notice also that the 4- 7 stageis a bit crowded. If you expand the width ofthis compartment, you will be able to movethe Q-8 stage down.

On the front panel, the BNC connectors arelocated 1 in from the case edge and are 1%on center from one another. The eight-pinmike-type connector is% in from the oppo-site edge of the case. Positioning of theseparts is not c ritical.

I had n o problem laying out the complicat-ed transmitter section. If you need morewidth, the 4 -1 2 adjacent stage has plenty ofroom an d can be moved further to the right.

A num ber of relatively long co nductor andRG-174 (% coaxial cable) runs enter andleave compartm ents that are not adjacent, butopposing. Also, some of the feedthrought e r m i n a ls a r e n o t b y p a s s c a p a c i t o rfeedthroughs. These also come out on theopposite side of the box (top or bottom).Carefully check to make sure that there isnothing in the way on the opposite sid e, andthat the feedthrough enters the proper stageon that side. The component layout drawingsrepresent relative positioning. In order tokeep the drawing clear, only major compo-nents have been drawn in.

Caution: The 4- 13 2N3866 transistor re-quires a top-hat heat sink. The case of thistransistor is connected to the collector and ittakes unkindly to being shorted, even for aninstant. Make sure the top hat clears otherparts when positioning this transistor on theboard.

Test EquipmentIn order to test and align you r transverter,

you will need the following test equipment.1. stable signal generator or calibrated os-

cillator and adjustable 50 ohm attenuatorcovering the appropriate RF range.

2. An electronic frequency cou nter coveringthe appropriate frequency range.3. A VH F range RF vacuum tube voltmeter

or a good DC VTVM with UH F R F probe.(Solid state is fine, too .)

4. VH F grid dip meter to check coil reso-nance. (Not absolutely necessary if youfollow coil winding data closely , but a realaid.)

5. A capacity bridge to confirm sm all valuesof capacity marked, and to set a givencapacity in a test circuit. (SeeHam RadioMarch 1980, page 54.)

6. A B ird Model 43 wattmeter or other power

measuring device, and a 50 ohm load(termination).7. A m ulti-range Volt-Ohm milliammeter.

Table 3 .7 Amateur Radio January, 1989

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GATE TO SOURCE VOLTAGE (VOLTS1

Figure 7 Drain current vs. gate-to-source voltageor the DV-1205S located in the low-levelampsection.

Coil Tap MysteryMany hams throw up their hands when it comes you might find the capacitor to have a much broad-

to determining the position of a tap on a coil. In er peak. Why is this? You have seen resonantmany cases, the tap impedance must match that of curves in handbooks. If the circuit Q is high, thethe following stage. Many hams who understand curve will have steep sides and a narrow band-why the tap is there, think, "That's fine. I can find width at some point down the curve, usually thethe proper point if only I know the base impedance half power point 3 dB). Measure the frequency atof the transistor, or the collector impedance." the half-power point on the low frequency side ofMathematical analysis can dissuade almost any- the peak and again on the high-frequency side.one, but don't despair -several aids are available. The difference in frequency will be the bandwidth.

The impedance Nomogram, ound in most hand- If the coil Q is low, the bandwidth will be greaterbooks, is practically indispensable for the RF than if ihe Q is high.builder. Better still are the cardboard reactance How do you vary Q by tapping the coil? We canslide rules usually found in technical book stores. assume that in this circuit (a receiver RF amplifier

AC or RF calculations are more complex. For stage) bou don't know the collector impedance orexampie, you might want a capacitor coupling for the base mpedance of the following stage. But youan RF circuit, but you don't know which size capac- do know that there will be two taps on the coil, anditor to use. You can determine the size by relating that thd taps will affect its Q and thus its selectivitythe capacitor resistance to DC. Here the slide rule or bandwidth. Here you have a coil and capacitoror Nomograph comes in handy. Since you know which hill resonate this circuit at the desired fre-the frequency and you know what would be a toler- quency. If the coil wire i s sufficiently large, and theable resistance, you can line up the arrow with the coil diameter is 13 to V2, the length of the coil , thefrequency and look at the resistance scale adja- circuit should have a high Q called "unloaded Q.cent to the capacitance scale. Select the capaci- The se~kctiv it~ ill be high and the bandwidth willtance that is opposite the lowest resistance you bevery narrow.want. Now you have to get this combination into the

We call the AC resistance "reactance" or circuit to do some work. Ground one side of the coil"impedance." In a resonant circuit, to find a coil to make the low impedance point. You want to feedimpedance, take the value of the capacitance that energy ihto the coil from the collector of the transis-resonates with this circuit, set the rule to this fre- tor of that stage. The collector impedance will loadquency, and look up the reactance. In a circuit at the Q of this coil. If the impedance is high, you can

resonance, the inductive and capacitive reac- connect it to the high end of the coil. The ideal pointtances are equal. is where the Q drops to one-half. Though the ideal

Often, instead of a tapped coil, you will find a is not always possible, at any rate the bandwidthcapacitive divider across the coil with the tap at the will increase. If the collector impedance is relative-junction of the two capacitors. The tota l capaci- ly low, you might wish to tap down the coil. Choosetance of two capacitors in series can be compared a point from l/4 to l/~Ilown. ollector impedance ofto two resistors in parallel (the product of each over a bipolar transistor is generally higher than thethe sum of each): base impedance of the following stage.

CI X c 2 Let's say you decide on a point 13 down theCST number of coil turns. Now comes the real loading,

CI +c2 getting energy from this tank circuit into the base ofAfter you get the resultant capacitance, Use the the following stage. The base tap will be much

slide rule to determine the reactance. You can find closer to the bottom or low impedance point of thethe reactance of each of the capacitors individual- coil. Place it 12 turn up from ground. Generally atly. See them as two resistors in a series across the this point it takes somewhat sophisticated equip-coil, and you can visualize the proportions of ment to dktermine what is truly happening.voltage across the two resistors, i.e., frequency is A sweep frequency signal generator, a calibrat-25 MHz. Two capacitors of 62 pF in series are ed markei.generator, and aspectrum analyzerareacross the coil. The reactance O the capacitors at used in the design laboratory. But when these arethat frequency is 100 ohms. The tap is at the 5 not available, there are other ways to make a satis-ohm point of the coil. DO not confuse this as the factory mbasurement. An RF vacuum tube volt-center of the coil, as the inductive reactance meter can really help.across the coil follows a square law rather than a Sending a steady signal into the receiver andlinear law. The reactance of the coil at this point is using some type of indicator, possibly an S-meter14 of the total. on the transceiver, the capacitor is tuned to reso-

nance. If the tuning is sharp, you know the band-oil Tap Loading width will be limited. Tapping the base further upThis is the meat of coil tapping You might wish on the coil will load the coil more heavily, and cut

to make some changes in the bandwidth of the down the selectivity, because the tuning will bereceiver section of your transverter. Possibly, if broader, and the receiver will cover a wider fre-you are interested in only aport ion of the band, you quency range. One point: the further down the coilmay want to sharpen the selectivity. (toward ground) that you place the collector tap,

Occasionally, when tuning a capacitor for reso- the more stable the circuit; but circuit gain will benance, you find a really sharp peak. Another time reduced..

Where To Find omponents at ReasonablePrices

In this project you will use quantities ofminiature plate capacitors, various sizes ofcompression capacitors, disc ceramic vari-ables and fixed capacitors, and dipped silvermicas (dog bone) compo nents. Fixed disc cer-amic of 0.001 mF are sprayed around thesource voltage lines as bypasses and as bothcoupling and decoupling circuits. In circuitsrequiring a degree of stability, use silver mica.Miniature PC board air variables, Johnson2-12 p F, usually plates total, are used instableRF circuits. These plates and small, highquality Arco compre ssiontype resonate coils.

You will notice lots of tuning capacitors. Thisadds up to a nice piece of change . Many of thes tages in the rece ive r cou ld be tunedtemporarily with a good-quality variable ca-pacitor. Remove the variable carefully so asnot to change its setting and m easure its valuewith a capacitance bridge. Now insert adipped silver-mica of that value in that spot.You can make minor adjustments by com-pressing or expanding the coi ls on thetoroidal core until resonance is re-estab-lished. T his is a m ildly complex substitution Ihave seen described in many publications.

All powdered iron and ferrite toroidalcores and baluns are available from AmidonAssociates. You can buy the eight-pin DINplug from Kenwood. Get two. The priceranges from 2 to 2.50 each.I purchased mycrystal from Jan Crystals 2400 CrystalDrive Fort Myers Florida 33906-9989. Or-der series resonant 0.001 percent accuracy

and enc lose a schematic diagram of the oscil-lator circuit.

TS 940lTransverter Jack Pin numbers

8-pin DIN pin num ber Function1 Ground2 PTT to transverter (12VDC@ 50 mA max.)

Ground4 TS-940 Final Amplifier d isable (Ground from transverter)5 Receive d (input) signal from tran sverter

ALC from TS-9 40 (not used here)Transm itted (output) signal to transverter

8 Internal TS-940 HF signal-connec t to pin 5to restore T S-940 HF when transv erter off.

publications. They also have crystals, tran-

Table 4.

4 7 Amateur Radio January, 1989

MHz Electronics Inc. 3802 N. 27th Ave. sistors, and Unelco noninductive capacitorsPhoenir Z 8501 7 advertises in most ham for the power amplifier. (Send for a catarog.)

Another excellent source for dipped silvermica capacitors, miniature variables, JFETs(15C), 4067 (25C) relays, and the small heatsink 2N3866 is Hosfelt Electronics Inc.261 0 Sunset Blvd. Steubenville OH 43952.The eight-pin miketype plug and jack are avail-able fromHenry Radio and Radio Shack.

Parts are also available at ham sw ap meets,and I have some items at very reasonableprices. If you have questions or comments,feel free to write me . Enclose an SASE for aresponse. Upon completing this project (Iwish to emphasize taking your time, step by

step) , the gratification and pride you w ill feelcannot be expressed on paper.The 220 power amplifier will appear in a

subsequent issue of73 Magaz ine .m